Many barrier materials and in particular amorphous polyamide resins are described in the literature. Thus, the subject of EP 411,601 is multilayer films for petrol tanks in which one layer is composed of amorphous copolyamide 6,I/6,T. However, these amorphous polyamide resins require high processing temperatures, for example during thermoforming of the finished article, and when the finished article containing these amorphous polyamide resins is biaxially orientated, its barrier properties are greatly reduced.
To overcome this disadvantage, provision was then made to combine amorphous polyamide resins with other resins, such as crystalline polyamides.
The subject of EP 287,839 is multilayer films in which one layer comprises a mixture of crystalline polyamide and of amorphous polyamide (PA-6,I/6,T).
EP 366,382 describes a composition for blown or moulded bottles composed of a mixture of PA-6, PA-6,6, of amorphous PA and of a lamellar filler, and WO 93/01108 describes a food container composed of a mixture of PA-6,6, of several amorphous PAs, of a lamellar filler and of an ionomeric resin.
These technical solutions have the disadvantage of requiring an additional stage, namely the mixing of more or less compatible resins, sometimes necessitating the addition of a compatibilizing agent.
Another very widespread barrier material is composed of the family of copolymers of ethylene and of vinyl alcohol (EVOH) . However, EVOH, in addition to its high price, has low resistance to moisture, which prevents it from being used in the preparation of single-material articles and, when it is used for composite articles, necessitates the addition of at least one intermediate layer between the EVOH and the wet medium, in order to prevent direct EVOH/wet medium contact. Moreover, when multilayer containers or films are prepared with an EVOH barrier layer according to a solid-phase thermoforming process, it is found that the EVOH cannot be shaped at the relatively low temperatures required for thermoforming the other resins of the multilayer structure without rupturing the EVOH layer, resulting in a loss in the overall barrier properties. In order to limit these disadvantages, it is also possible to combine the EVOH with other resins. Thus, the subject of EP 305,146 is mixtures of EVOH (mainly) and of amorphous polyamides for the manufacture of multilayer containers obtained by thermoforming.
However, none of the abovementioned references provides a material which simultaneously possesses sufficient barrier properties to be used alone and is capable of being employed easily and processed, in particular according to thermoforming, drawing techniques, without a decrease in its barrier properties.